It is well known in the an to employ blowing or foaming agents to lighten thermally processed polymers (TPPs) such as thermoplastic resins, thermoset resins and thermoset elastomeric compounds.
It is advantageous to employ these blowing agents with TPPs when lighter parts are desired, or when other properties, such as compressibility or greater flexibility as in the case of an automotive weather stripping, are desired. These blowing agents are added to the feed at the beginning of or during the process cycle and perform their designed function by decomposing under the temperature conditions employed during thermal processing thereby producing a gas as a by-product of this decomposition. Bubbles of gas thus formed are entrained within the TPP, and when the TPP cures or hardens, the bubbles become fixed in the pan produced.
Lower cost bicarbonates which generate CO.sub.2 are employed as blowing agents, but the results achieved during their thermal decomposition are relatively unpredictable, and therefore they are not usually employed to produce high quality parts. The more predictable azodicarbonamides (azides) or similar organic compounds are more commonly used. During the thermal decomposition of these azides, nitrogen gas, along with lesser amounts of other gases, is produced.
Although considered effective, azides, in actual practice, are relatively poor performers in that only a portion of the nitrogen gas produced by the compound is actually employed in blowing or expanding the TPP. This is due to the fact that gas capture in the TPP only occurs during a specific time interval in the processing operation.
In the case of a thermoset, the TPP will only capture and hold gas bubbles during a relatively short interval in its processing. It only begins to capture gas when it has cured sufficiently to yield the necessary film strength and viscosity required for gas capture, and it stops capturing gas when its film strength and viscosity exceed a limit wherein the cells rupture and no longer can take advantage of additional gas generation. Unfortunately, the short interval during which gas capture is possible is only a fraction of the interval during which azides actually generate gas. Thus the portion of gas actually captured is only a portion of the total of the nitrogen gas actually produced by the azide. In an attempt to overcome this inefficiency, azide accelerators such as triethanolamine, zinc stearate, lead stearate, barium stearate, calcium stearate, zinc oxide and dibasic lead phthalate are often employed. These accelerators can lower the temperature at which azides begin to decompose, but the inefficient relationship between the gas capture interval and the gas evolution interval still exists, and the addition of heavy metals is undesirable.
This inefficiency of azides causes many problems. Blown compounds cost more to produce because azides have to be overused to achieve a specific density in the finished part or sheet. Also some of the unblown azide in the finished part may eventually gas off, causing post-blow problems such as the surface blistering of painted parts, and objectional odors in the part or sheet at the consumer level. Post-blow is also a problem if a TPP containing unreacted azide is recycled, in that the azide may decompose during the recycling process.
Additionally, compounded materials containing azides have a limited shelf life between the time they are mixed and the time they are thermally blown. This shelf life may be as short as or shorter than 24 hours, and batches exceeding shelf life must be discarded or reworked.
Another problem caused by the use of azides is the fact that some of the azide decomposition products which remain in the material being blown are potentially reactive, and can, over time, react with, and subsequently degrade the material of the finished parts in which they are contained. This effect is greatly accelerated when the parts are normally subjected to high temperature conditions during their normal lifetime. Additionally, when processing azide blown TPPs in a heated screw extruder, cyanuric acid, an azide by-product, tends to plate out on the unit's internal surfaces, causing production problems as well as corrosion problems.
Another significant problem encountered with azide s in the blowing of thermo set elastomers is the great difficulty with which thick parts are produced. The blowing of thermoset elastomers with azides involves both the generation of blowing gas through thermal decomposition of the azide as well as the curing of the compound by heat. To effectively produce a blown part, gas generation must occur during that portion of the curing cycle in which the compound viscosity and film strength are high enough to trap gas bubbles, yet not so high as to inhibit blowing or to cause rupture of the individual cells. Proper selection of azide and compound cure rate will allow the production of blown parts of relatively narrow thicknesses by heat treatment in an oven, heated mold or molten salt bath. When a thicker part is desired, however, a major problem is encountered when the part is heated wherein the outermost layers of the part reach reaction temperatures before the interior layers, and these outer layers tend to blow and cure, hindering the conduction of heat to the interior of the part, and also inhibiting the blowing of the interior of the part due to the formation of the cured, less pliable outer skin. Because azides are not microwave receptive, their thermal decomposition can only be accomplished through the aforementioned conduction of heat from the outer surfaces to the inner portions of the part, thereby causing difficulty in the production of thick parts.
Azides also tend to discolor light colored TPPs, and overuse tends to exaggerate this problem. The most significant problem with azides, however, may be the potential for the creation of nitrosamines when azides are used as a blowing agent. Azide overuse increases this potential nitrosamine formation.
U.S. Pat. No 4,029,612 discloses azodicarbonamide blowing agents containing silica which prevent plate-out onto machine parts of cyanuric acid which forms as a result of azide decomposition, which silica also provides 1.5 to 20 parts of water to 100 parts by weight azodicarbonamide. Additional water may be added to the composition by adding other compounds containing available water as long as no more than a total of 20 parts of water per 100 parts of azodicarbonamide are present, and these compounds may consist of silicates, oxides, chlorides, carbonates, sulfates, hydroxides, tanrates, phosphates, benzoates, borates and oxalates. The purpose of the additional water which may be added through the use of these compounds is to prevent cyanuric acid plate-out. U.S. Pat. Nos. 4,286,070, 4,278,768 and 4,266,038 and Canadian Patent No. 844652 describe the use of water or of hydrated salts as blowing compounds, including the highly alkaline sodium metasilicate pentahydrate. These blowing compounds are limited to use in narrowly defined specific polymers or combinations of these polymers and lower alkalinity alkali metal silicates which intumesce are not discussed. U.S. Pat. No. 2,911,382 also describes the use of hydrated compounds which must be used in conjunction with organic liquid blowing agents such as acetone or alcohol which serve as blowing agents for narrowly defined polymers. In U.S. Pat. Nos. 4,719,249, 4,686,244, and 4,521,333, fire retardant copolymers are described which contain fillers consisting of hydrated compounds including alkali metal silicates to impart fire resistance. These compounds are basically fire proofing compounds and are designed to function under intense heat and fire conditions at temperatures higher than the thermal decomposition temperature of the polymer base. U.S. Pat. No. 5,369,147 teaches the use of water absorbed into fillers as a blowing agent for narrowly defined polymers. In U.S. Pat. No. 3,511,787 a reactive, phylloidal, high surface area silica containing water is described as an activator/promotor for chemical blowing agents. U.S. Pat. No. 3,743,605 describes the use of compounds including sodium and potassium silicates as inert dispersing agents for blowing agents such as azodicarbonamide, which inert compounds do not decompose to form a gas when the polymer composition in which they are incorporated melts or fuses.
The above described references do not disclose or teach the novel blowing agents of the present invention which are readily distinguishable from those of the prior art.